The present invention relates to harvesters having a metal detector thereon, and is more particularly concerned with such an arrangement wherein the metal detector is operative to produce a high frequency oscillating field which is interrupted by metal objects that approach the field, so that the field interruption produces a signal which can be used to either control or shut down the operation of the harvester.
Harvesters having metal detectors thereon have been suggested heretofore. In one known arrangement of this general kind, the metal detector comprises a detector coil which is mounted on a wooden beam that is positioned at a distance that is substantially one meter forward of the foremost metal part of the harvester. The detector coil reacts to the presence of metal pieces to produce an acoustic or visual alarm so that the harvester can be shut down before the detected metal object is actually transported into the harvester. Harvester arrangements of this type exhibit the disadvantage that their overall length is increased by the wooden beam and detector coil mounted thereon. In addition, these known systems have the disadvantage that a signal may be generated by metal objects that are actually embedded in the ground and which would not normally be a cause of concern, this disadvantage arising from the fact that it is normally impossible to so position the metal detector that it will be sensitive only to metal objects at ground level.
Other harvesters have been suggested heretofore (e.g., as described in DE-OS No. 22 52 595 and DE-OS No. 25 52 805) which are provided with metal detectors of the magnetic type, i.e., they employ arrangements for generating a direct current magnetic field, and an associated magnetic field detector coil. Magnetic metal pieces which interrupt the direct current magnetic field break the magnetic lines of force and thereby produce a voltage impulse in the detector coil which acts as a signal that is used to control an associated electrical switch. A fundamental disadvantage of this type of detector system is that it can detect only the presence of ferromagnetic objects, and it is incapable of protecting the harvester from damage resulting from the transport into the harvester, along with the harvested material, of other types of metal objects, e.g., those fabricated of copper, brass or aluminum.
In an effort to obviate some of the foregoing disadvantages, it has been suggested heretofore that agricultural harvesters be provided with an acoustic scanning system to detect foreign objects which might cause damage. This type of arrangement, however, can be used effectively only when the harvester operates on flat ground surfaces because, when the system is operated on uneven land, roots, dirt and other foreign bodies are detected and cause undesired activation of the apparatus. In addition, this particular system is incapable of protecting the harvester against the ingress of foreign bodies which are firmly entangled in the harvest material since an acoustic signal indicating the presence of the foreign body will be given only when the foreign body is located adjacent the outer surface of the material flowing toward and into the harvester.
The present invention is directed to an improved metal detector arrangement which combines the advantages of approaches suggested heretofore without exhibiting their disadvantages and, more particularly, is concerned with the provision of a harvester/metal detector arrangement which is compact, which is insensitive to metal elements that are embedded in the ground, and which protects the harvester against damage resulting from the presence of both magnetic and non-magnetic metal objects that are supplied to the harvester along with the harvest material.